1. Field of the Invention
This invention relates to electronic systems and more specifically to circuit boards for electronic systems.
2. Description of the Related Art
Electronic systems such as computer systems inherently generate Electromagnetic (EM) fields. All EM fields are originally sourced by the active electronic components in the system, but are delivered into space as Electromagnetic Interference (EMI) by various system structures which act as antennas. Three sources of EMI in a computer system are:
1) Circuit board--differential and common mode EM fields from circuit board traces and planes which may also be referred to as back-plane radiation.
2) Cabling--differential and common mode EM fields from flex circuits, internal cabling, or external cabling.
3) Integrated circuits(ICs)--EM fields associated with IC(s). Each source contributes to the overall EMI emissions profile of a computer system.
A type of EMI radiation generated by a circuit board during the operation of a computer system is commonly referred to as transverse electromagnetic mode (TEM) radiation or TEM waves. During the operation of a computer system, RF currents are generated by sources such as alternating clock currents transmitted through the circuit board traces and by instantaneous current changes in the electronic board components such as IC chips. These complex RF currents travel along and between the power planes and produce EM emissions at the circuit board edges, some of which are emitted as TEM waves. TEM wave radiation is generated from these RF currents through differential voltage disturbances between two or more power planes embedded within the circuit board. The RF differential voltage at the power plane edges generates electromagnetic waves through the air to ground following a co-linear, parallel plate, transmission line model, with fringing occurring at the open circuit end of the parallel plate transmission line.
An electromagnetic interference (EMI) shield (e.g. a metal chassis) in a computer system is designed to contain EM waves produced by EM sources in the computer system and/or to exclude external EM waves from external EM sources. With a typical EMI shield, radio frequency (RF) electrical currents originating from the circuit traces and electronic components induce EM waves or fields that are absorbed by the shield (A), reflected by the shield (R). The rest of the EM waves are transmitted by the shield. Shielding effectiveness (SE), which is expressed in dB, follows: EQU SE.sup.(dB) =A.sup.(dB)+ R.sup.(dB). EQU SE.sup.(dB) =20 log F1/F2.sup.(dB).
where F1 is the wave field strength of the EM source, and F2 is the wave field strength of the portion re-transmitted by the shield.
The higher the EM fields generated by the system, the higher the required SE to reduce the portion of the EM fields re-radiated by the shield. Increasing the SE typically is accomplished by increasing the thickness of the metal panels of a metal shield or increasing the thickness of metallic coating on the outer plastics of the computer system, either of which increases the cost, complexity, and/or weight of the system. Furthermore, designing a shield with a high SE requires interface structures built within high tolerances to prevent EM waves from leaking through the shield via openings in the shield such as openings for receiving external data and power signals. These high tolerance interface structures also add to the cost and/or complexity of a computer system. Because of these and other disadvantages, it is desirable to reduce the EM radiation generated by the electronic system on or within a circuit board.
The Federal Communications Commission (FCC) in the United States and other international agencies set standards for the maximum intensity of electromagnetic energy which can be radiated and conducted in specific electromagnetic bands from a computer system. Accordingly, it is desirable to reduce the TEM waves generated from the edge of a circuit board in order to reduce the overall EMI emissions generated by a computer system or other electronic system during its operation.